Analog-to-digital (A/D) converter circuits are known to comprise devices capable of converting analog wave forms into corresponding digital representations. Sigma-delta A/D converters are one type of A/D circuit known in the art. In a sigma-delta A/D converter, the analog wave form is input to an integrating lowpass filter, the output of which serves as input to a one-bit quantizer. The output of the one bit quantizer serves not only as the digital output of the sigma-delta A/D converter, but it is also fed back and summed with the analog input to produce a difference signal at the input of the integrating lowpass filter. An error signal is produced as the integrating lowpass filter operates on the difference signal. In essence, the sigma-delta A/D converter attempts to produce digital representations of the input signal such that the error signal is continually minimized. Ideally, the resulting power spectral density of the digital representation substantially matches that of the analog input, with the addition of quantization noise. The spectral shape of the quantization noise is greatly influenced by the frequency characteristics of the integrating lowpass filter.
Baseband sigma-delta A/D converters have a property that with certain integrator time constants, some input levels, e.g., certain dc input levels, can cause low-level tones to be present in the passband of the sigma-delta output, which can exhibit spurious outputs in the form of unwanted spectral components to be present when the signal is FM (frequency modulation) demodulated. Such tones are undesirable for a radio receiver, as the intended recovered signal will quite often be distorted and difficult to distinguish from noise. These tones can produce other deleterious effects depending upon the A/D converter's application.
A similar phenomenon occurs in other A/D converters, such as Flash converters, wherein passband tones can be produced with an input signal present combined with certain dc input levels. These tones are equally undesirable.
Typically, the integrating lowpass filter utilizes multiple integration stages to realize effective out-of band attenuation of quantization noise. A cost associated with the use of higher order integrating filters is their inherent instability. When an analog input becomes excessively large, this instability can result in uncontrollable oscillations at the output of the sigma-delta A/D converter, and hence, in the loss of information contained in the analog input. Only when the input has been sufficiently reduced will the oscillations cease and normal operations continue. This situation can be resolved in several ways. Rather than using multi-stage integrating filters, a single stage (single pole) filter can be used. Single pole integrating filters, referred to as first order filters, guarantee stability but do not offer sufficient noise suppression performance. Alternatively, the pole and zero location of the multiple integration stages can be altered to minimize instability, but at the expense of inferior spurious output performance.
Accordingly, there is a need for a sigma-delta A/D converter that reduces unwanted spectral components from the sigma-delta output without resulting in uncontrollable oscillations or poor noise suppression performance.